Immunocytochemical localization of luteinizing hormone-releasing hormone in the brain of the goldfish Carassius auratus

A 45-years journey within the reproductive brain of fish

This article summarizes the scientific carrier of Dr. Olivier Kah, currently emeritus research director at the National Center of Scientific Research (CNRS) in France. Olivier Kah partly grew up in Africa where he developed a strong interest for animals. He studied biology in Paris and Bordeaux. He next received his PhD at the University of Bordeaux en 1978 and his Doctor of Science degree in 1983. He joined the CNRS in 1979 until his retirement in 2016. Olivier Kah dedicated his carrier to the study of reproduction, in particular to the roles of brain neuropeptides and neurotransmitters in the control of the reproductive axis in vertebrates, mostly fish. More specifically, Olivier Kah was specialized in the use of morphofunctional techniques that he implemented to the study of the organization of the hypothalamo-pituitary complex. He was also interested in the steroid feedback and studied intensively the expression and regulation of estrogen and glucocorticoid receptors in the rainbow trout and the zebrafish. In the last 10 years, Olivier Kah's team focused on the expression and regulation of aromatase in the brain and established that aromatase expression is restricted to a unique brain cell type, the radial glial cells, which serve as progenitors during the entire life of fish. He is also interested in the impact of endocrine disruptors using the zebrafish as a model and recently his team has developed an exquisitely sensitive in vivo assay to screen estrogenic chemicals on zebrafish embryos.

Distribution of endomorphin-like-immunoreactive neurones in the brain of the cichlid fish Oreochromis mossambicus

Endomorphins (EMs) are tetrapeptides involved in pain and neuroendocrine responses with a high affinity for μ-opioid receptors in mammals. In the present study, we investigated the distribution of EM-like-immunoreactive (EM-L-IR) neurones in the brain of the cichlid fish Oreochromis mossambicus. Application of antisera against EM-1 and 2 (EM-1-2) revealed the presence of EM-L-IR somata and fibres throughout the different subdivisions of the olfactory bulb, such as the olfactory nerve layer and the granule cell layer. Although the extensions of EM-L-IR fibres were seen along the medial olfactory tract, intensely labelled EM-L-IR somata were found in different subdivisions of the telencephalon. In the diencephalon, intensely stained EM-L-IR neurones were noted in the preoptic area, the nucleus preopticus pars magnocellularis, the suprachiasmatic nucleus, the nucleus lateralis tuberis pars lateralis and the nucleus lateralis tuberis pars medialis regions, whereas projections of EM-L-IR fibres were also seen along the hypothalamic-hypophyseal tract, suggesting a possible hypophysiotrophic role for these neurones. Intense to moderately stained EM-L-IR neurones were noted in different subdivisions of thalamic nucleus, such as the dorsal posterior thalamic nucleus, commissura posterior, ventromedial thalamic nucleus, nucleus posterior tuberis, ventrolateral thalamic nucleus and medial preglomerular nucleus. Numerous intensely stained perikarya and axonal fibres were also noted throughout the inferior lobe, along the periventricular margin of the reccessus lateralis and in the nucleus recesus lateralis regions. In addition, numerous moderately labelled EM-like neuronal populations were found in the secondary gustatory nucleus and rostral spinal cord. The widespread distribution of EM-L-IR neurones throughout the brain and spinal cord indicates the diverse roles for these cells in neuroendocrine and neuromodulatory responses for the first time in fish. The present study provides further insights into the possible existence of EM-like peptides in early vertebrate lines and suggests that these peptides might have been well-conserved during the course of evolution.

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Neuropeptide Y in the olfactory system, forebrain and pituitary of the teleost, Clarias batrachus

Distribution of neuropeptide Y (NPY)-like immunoreactivity in the forebrain of catfish Clarias batrachus was examined with immunocytochemistry. Conspicuous immunoreactivity was seen in the olfactory receptor neurons (ORNs), their projections in the olfactory nerve, fascicles of the olfactory nerve layer in the periphery of bulb and in the medial olfactory tracts as they extend to the telencephalic lobes. Ablation of the olfactory organ resulted in loss of immunoreactivity in the olfactory nerve layer of the bulb and also in the fascicles of the medial olfactory tracts. This evidence suggests that NPY may serve as a neurotransmitter in the ORNs and convey chemosensory information to the olfactory bulb, and also to the telencephalon over the extrabulbar projections. In addition, network of beaded immunoreactive fibers was noticed throughout the olfactory bulb, which did not respond to ablation experiment. These fibers may represent centrifugal innervation of the bulb. Strong immunoreactivity was encountered in some ganglion cells of nervus terminalis. Immunoreactive fibers and terminal fields were widely distributed in the telencephalon. Several neurons of nucleus entopeduncularis were moderately immunoreactive; and a small population of neurons in nucleus preopticus periventricularis was also labeled. Immunoreactive terminal fields were particularly conspicuous in the preoptic, the tuberal areas, and the periventricular zone around the third ventricle and inferior lobes. NPY immunoreactive cells and fibers were detected in all the lobes of the pituitary gland. Present results describing the localization of NPY in the forebrain of C. batrachus are in concurrence with the pattern of the immunoreactivity encountered in other teleosts. However, NPY in olfactory system of C. batrachus is a novel feature that suggests a role for the peptide in processing of chemosensory information.

mRNA expression of gonadotropin-releasing hormones (GnRHs) and GnRH receptor in goldfish

In goldfish (Carassius auratus), two distinct forms of gonadotropin-releasing hormone (GnRH), namely, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), have been identified in the brain using chromatographic, immunological, and molecular cloning approaches. These two native GnRHs act on specific receptors in the anterior pituitary to stimulate the synthesis and release of gonadotropins and growth hormone in goldfish. To evaluate the potential roles of sGnRH and cGnRH-II in both neural and reproductive tissues in goldfish, we studied the mRNA expression of sGnRH, cGnRH-II, and GnRH receptor (GnRH-R) in discrete brain areas, pituitary, ovary, and testis by a combined reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot analysis. Total RNA was extracted from various tissues of sexually recrudescent male and female goldfish and RT-PCR was performed with primers specific for GnRH-R complementary DNA (cDNA), sGnRH cDNA, cGnRH-II cDNA-1, and cDNA-2. Results showed that GnRHs and GnRH-R mRNAs are differentially distributed in the brain. In the goldfish brain, sGnRH mRNA was predominantly expressed in the forebrain areas (olfactory bulb, telencephalon, and hypothalamus) whereas cGnRH-II mRNA-1 were expressed in all brain areas including olfactory bulbs and optic tectum-thalamus. The expression level of cGnRH-II mRNA-2 was much lower than that of cGnRH-II mRNA-1 in the brain. On the other hand, GnRH-R mRNA was expressed in all brain regions and pituitary. In the ovary and testis, GnRH-R mRNA, sGnRH mRNA, and cGnRH-II mRNA-1, but not cGnRH-II mRNA-2, are expressed. Sequence analysis of the PCR products showed that nucleotide sequences of GnRH-R in gonads are identical with that in the brain and pituitary. The coexistence of GnRHs and GnRH-R mRNAs in both neural and gonadal tissues supports the notion that sGnRH and cGnRH-II may act as neurotransmitters and/or neuromodulators in the brain and as autocrine and/or paracrine hormones in gonadal tissues in addition to their established neuroendocrine roles at the pituitary of goldfish.

Localization of salmon gonadotropin-releasing hormone mRNA and peptide in the brain of Atlantic salmon and rainbow trout

The decapeptide gonadotropin-releasing hormone (GnRH) is a key hormone for the central regulation of reproduction. The distribution of salmon GnRH (sGnRH), which is the major form in salmonids, has been studied in different fish species by immunocytochemistry. Discrepancies in data concerning the distribution of sGnRH perikarya led us to investigate this problem in two species, the Atlantic salmon and the rainbow trout, with in situ hybridizaiton of sGnRH messenger, a highly specific molecular tool. By Northern blot analysis, the rainbow trout sGnRH messenger appears to be about 500 bases in length, which is close to those isolated from Atlantic salmon or masu salmon and characterized previously. In situ hybridization with riboprobes generated with Atlantic salmon sGnRH cDNA demonstrated that sGnRH perikarya are restricted to the ventral part of olfactory bulbs, telencephalon, and preoptic area. They are distributed on a nearly continuous line extending from the olfactory bulbs to the preoptic area in both salmonid species studied. Despite the presence of GnRH-like immunoreactivity in the preoptic magnocellular nucleus (NPOm) and in the tegmentum of the midbrain (MT), the sGnRH mRNA is not present in these two structures. Stained cells in NPOm could be target cells for GnRH and immunoreactive neurons in MT are likely to be chicken GnRH-II containing cells. Our study not only gives a precise distribution of the sGnRH system in two salmonids, Atlantic salmon and rainbow trout, but also clarifies the ambiguous data published up to now in rainbow trout.

Targeting of gonadotropin-releasing hormone axons from preoptic area grafts to the median eminence

Implantation of normal GnRH neurons can reverse many of the reproductive deficiencies that characterize hypogonadal (hpg) mice. Since the GnRH axons follow a stereotyped trajectory to their target we investigated the possibility that host brain regions adjacent to the graft might provide signals that induced this directional growth. The role of the adenohypophysis in GnRH axonal outgrowth was studied in mice with co-grafts of fetal preoptic area (POA) and pituitary and in hypophysectomized hosts. When fetal pituitaries were grafted together with the POA, immunoreactive GnRH fibers did enter the glandular tissue but they also grew into the host median eminence. Surgical removal of the pituitary of hpg hosts prior to POA graft placement was also compatible with GnRH innervation of the host median eminence although in some individuals that innervation pattern was confined to the more caudal aspects. The results of these two experiments suggest that the anterior pituitary gland may be an attractive target for GnRH axons but that this tissue is not essential for directed GnRH axonal outgrowth to its target. To determine if the median eminence itself could direct the growth of GnRH axons, co-grafts of POA and a fetal medial basal hypothalamic (MBH) block, which was predominantly median eminence, were made. Immunocytochemistry showed that an intragraft mini-median eminence was formed with a highly organized and robust GnRH innervation. Ultrastructural analysis indicated that these axons terminated near fenestrated capillaries. However, even under these conditions some GnRH axons exited into the host median eminence. It now seems likely that a cellular component of the median eminence can provide a signal to attract GnRH axons. Whether this signal is produced by the specialized ependymal cells, by the endothelia, or by meningeal (pial) components must now be tested.

Annual variations in brain serotonin and related compounds of domesticated rainbow trout (Oncorhynchus mykiss)

The levels of tryptophan (Trp), 5-hydroxytryptamine (serotonin, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in two brain regions (hypothalamus and medulla oblongata) of rainbow trout (Oncorhynchus mykiss) were measured throughout the year using a sensitive high performance liquid chromatographic method with electrochemical detection. Trp was also quantified in serum, liver and brain regions. Trp concentrations were higher in sera than in tissues (brain and liver) throughout the year. Hypothalamic and medullary Trp offered similar annual patterns showing two peaks, the first on 10 May and the second on 15 June. 5-HT levels were always higher in the hypothalamus than in the medulla and much higher than the 5-HIAA levels in both regions. Moreover hypothalamic and medullar serotonin patterns were very different, in the latter region few variations were observed. In addition, the 5-HIAA/5-HT ratio was different in the brain regions, the hypothalamus showed two peaks (April 8 and June 15) whereas the equivalent was not observed in the medulla. All measured compounds showed significant variations during the year.

Immunocytochemical demonstration of salmon GnRH and chicken GnRH-II in the brain of masu salmon, Oncorhynchus masou

We have recently developed sensitive and specific radioimmunoassays (RIAs) for salmon gonadotropin-releasing hormone (sGnRH) and chicken GnRH-II (cGnRH-II) and have measured the contents of both GnRHs in the rainbow trout brain. Our results showed that contents of the two GnRHs are variable among different brain regions. Therefore, in order to confirm the differential distribution of the two GnRHs by a different technique, we examined the distribution of immunoreactive sGnRH and cGnRH-II in the brain of masu salmon by using immunocytochemical techniques. sGnRH immunoreactive (ir) cell bodies were scattered in the transitional areas between the olfactory nerve and the olfactory bulb, the ventral olfactory bulb, between the olfactory bulb and the telencephalon, the ventral telencephalon, and the preoptic area. These sGnRH-ir cell bodies were dispersed in a strip-like region running rostrocaudally in the most ventral part of the ventral telencephalon. sGnRH-ir fibers were distributed in the various brain regions from the olfactory bulb to the spinal cord. They were especially abundant in the olfactory bulb, ventral telencephalon, preoptic area, hypothalamus, deep layers of the optic tectum, and thalamus. sGnRH-ir fibers also innervated the pituitary directly. cGnRH-II-ir cell bodies were found in the nucleus of the medial longitudinal fasciculus (nMLF). The distribution of cGnRH-II-ir fibers was similar to that of sGnRH-ir fibers, except that cGnRH-II-ir fibers were absent in the pituitary. The number of cGnRH-II-ir fibers was much fewer than that of sGnRH-ir fibers. The results of the present immunocytochemical study are in basic agreement with those of our previous RIA study. Thus, we suggest that in masu salmon, sGnRH not only regulates gonadotropin (GTH) release from the pituitary but also functions as a neuromodulator in the brain, whereas cGnRH-II functions only as a neuromodulator.

Hypothalamic transplantation repair of reproductive defects in hypogonadal mice

The defect of the hypogonadal mouse, resulting in infantile reproductive organs and severely reduced gonadotropin levels, is due to a truncation of the gene encoding for preprogonadotropinreleasing hormone. The hypogonadal mouse bearing a graft containing normal gonadotropin-releasing hormone neurons may show testicular development, seminal vesicle growth, and increased gonadotropin production in males. Normalization of gonadotropin levels in females is frequently associated with the capacity for a reflex ovulation followed by pregnancy and bearing of live young. All of these phenomena are dependent on the outgrowth of gonadotropin-releasing hormone axons from the graft to the host median eminence and the hypophysial portal capillaries.

Gonadotropin-releasing hormone (GnRH) immunoreactive system in the brain of the dwarf gourami (Colisa lalia) as revealed by light microscopic immunocytochemistry using a monoclonal antibody to common amino acid sequence of GnRH

The present paper aims to give a morphological basis for the study of the terminal nerve system and its relation to the whole gonadotropin-releasing hormone (GnRH) immunoreactive (ir) neuronal system. We examined the GnRH-ir neuronal system of a tropical fish, the dwarf gourami, by using a recently developed monoclonal antibody against GnRH (LRH13) which recognizes the amino acid sequence common to all known variants of GnRH (Park and Wakabayashi, Endocrinol. Jpn. 33:257-272, '86). The ganglion cells of the terminal nerve (TN-ggl cells) in the transitional area between the olfactory bulb and the telencephalon reacted strongly with the LRH13. A distinct bundle of axons emanating from the TN-ggl cells ran caudally through the ventral telencephalon and the preoptic area. Some of these axons entered the optic nerve and innervated the retina. The remaining axons continued caudally to enter the hypothalamus and the midbrain. A second group of GnRH-ir cell bodies was found in the preoptic area. A distinct bundle of GnRH-ir fibers originating from these cell bodies innervated the pituitary. This pathway is equivalent to the preoptico-infundibular pathway of other vertebrates, and the GnRH in this pathway is presumed to function as hypophysiotrophic hormone to facilitate the release of gonadotropins from the pituitary. The distribution of GnRH-ir fibers in the brain was extensive. Most fibers apparently originated from the TN-ggl cells and covered various brain regions from the olfactory bulb to the spinal cord. They were especially abundant in the olfactory bulb, ventral telencephalon, preoptic area, optic tectum, and some hypothalamic areas. Thus, GnRH might function as a neuromodulator and/or neurotransmitter in these areas. The abundant GnRH-ir fibers in the ventral telencephalon and the preoptic area might affect some aspects of sexual behavior, since these areas have been suggested to be involved in the control of sexual behavior in teleosts.

Corticotropin-like immunoreactivity in the brain and pituitary of three teleost species (goldfish, trout and eel)

Immunostaining of brain and pituitary sections of three teleost species (goldfish, trout and eel) with antisera to porcine and human ACTH 1-39 revealed the presence of an ACTH (adrenocorticotropic hormone)-like peptide in the ventral hypothalamus. Perikarya were localized in the rostral, median and posterior portions of the nucleus lateralis tuberis (NLT); some were in contact with the cerebrospinal fluid. A dense network of immunoreactive (ir) fibers occurred in the peri-infundibular region and extended into the periventicular tissue, around the lateral and posterior recesses. Rostrally directed ir-fibers reached the telencephalon either ventrally or mediodorsally; some were observed in the olfactory lobe. In the mesencephalon, ir-fibers penetrated into the optic tectum of the goldfish. In the pituitary, both antisera intensely labeled rostral ACTH cells. Small groups of labeled cells were scattered in the rostral pars distalis and the proximal pars distalis. A gradient of activity was evident among ACTH cells: those located along the rostral neurohypophysis containing corticotropin-releasing factor nerve terminals were larger and often more marked than those farther away from the neural tissue. ACTH-like peptide in the brain may act as a neuromodulator, mainly in the NLT and the preoptic nucleus, and around the nuclei of the ventricular recesses containing serotonin and catecholamines.

Immunohistochemical localization of thyrotropin-releasing hormone in the brain of carp, Cyprinus carpio

The localization of immunoreactive thyrotropin-releasing hormone (IR-TRH) in the forebrain and pituitary of carp was studied immunohistochemically using the peroxidase-antiperoxidase technique. In the hypothalamus. IR-TRH was present in the neuronal processes extending from the preoptic nucleus (NPO) to the nucleus recessus lateralis (NRL). Cell bodies appeared to be present in the inside of the medial NRL. Most of these neurons were fusiform and bipolar. Immunoreactive-beaded fibers streamed from the anterior part of the NRL toward the nucleus posterioris periventriculas and nucleus lateral tuberis pars posterioris. Vertical strands of the beaded fibers ran in the nucleus lateral tuberis pars anterioris. In the pituitary, the reaction product was found in the neural lobe, where intense immunoreactivity was evident along neural fibers entering the intermediate lobe. Staining could be detected only rarely in the anterior lobe. IR-TRH-beaded fibers were present in the olfactory stalk as well as in the caudal and inner parts of the olfactory bulb. In contrast to the high concentration of IR-TRH in the olfactory bulb, immunohistochemical data from this work indicated weak immunoreactivity in this region.

Existence of a GnRH immunoreactive nucleus in the dorsal midbrain tegmentum of the chameleon

The GnRH system of the chameleon brain was studied at light microscopic and ultrastructural levels by use of an immunohistochemical technique with antibodies directed against salmon gonadotrophin-releasing hormone. Immunoreactive (IR) perikarya were found in the anterior midbrain tegmentum. At this level numerous IR cell bodies were detected around the fasciculus longitudinalis medialis (FLM). The more rostral neurons were observed dorsal to the FLM and progressively tended to be lateral to it along the midline. More caudally, they were found ventral to the FLM. At the electron microscope level, these cells were seen to contain large granular vesicles and to receive numerous synaptic inputs. A prominent pathway was traced from these cell bodies along the medulla oblongata to the spinal cord. A second IR pathway ascended rostrally to the habenular complex. No IR perikarya were located in the anterior brain including the olfactory bulbs.

Catechol-O-methyltransferase in the brain of the male African catfish,Clarias gariepinus; distribution and significance for the metabolism of catecholestrogens and dopamine

Catechol-O-methyltransferase, involved in the methylation of catechol substrates, was localized in the brain of the male African catfish,Clarias gariepinus, by means of a radiometric assay using [Methyl-(3)H]S-adenosylmethionine as methyldonor and catecholestrone as substrate. Fore- and midbrain were divided into eighteen, 500 μm thick, transverse sections. With a hollow needle (diameter 1 mm), specific areas of the brain were punched out and assayed. The catechol-O-methyltransferase activity was calculated from the amount of radioactive methoxyestrone formed from catecholestrone and expressed in pmol.mg(-1) tissue.h(-1).The enzyme could be demonstrated throughout the brain. Although the enzyme activity did not differ very much between the various brain regions (max. 15.4; min. 7.5 pmol), there were some areas in the brain with a more than average activity,i.e., the lateral telencephalon (10.3 pmol), the nucleus preopticus (13.1 pmol), nucleus lateralis tuberis (11.0 pmol) and nucleus recessus posterioris (12.0 pmol) of the diencephalon, the tectum opticum (15.4 pmol) and torus semicircularis (13.6 pmol) of the mesencephalon, and the caudal cerebellum of the metencephalon (10.8 pmol). The lowest activity was detected in the caudal metencephalon (7.5 pmol).The presence of the enzyme catechol-O-methyltransferase in the brain of the African catfish and the observation that both catecholestrogens and dopamine can be methylated by this enzyme suggest that catecholestrogens can influence the methylation (inactivation) of dopamine. Incubations of forebrain homogenates with dopamine and catecholestrone or catecholestradiol confirmed that both catecholestrogens can inhibit the methylation of dopamine. Lineweaver-burk plots with various concentrations of the catecholestrogens indicated that the inhibition is competitive. Dixon plots from the inhibition studies gave inhibition constants of 1.4 and 0.6 μM for catecholestrone and catecholestradiol, respectively, indicating that catecholestradiol is a two times stronger inhibitor than catecholestrone.The significance of the inhibition of the methylation of dopamine by the catecholestrogens in the brain is discussed in the light of the negative feedback of gonadal steroids on the central regulation of reproductive processes.

Immunocytochemical localization of LH-RH in the brain and pituitary of the catfish, Clarias batrachus (Linn.)

An elaborate organization of luteinizing hormone-releasing hormone (LH-RH) immunoreactive (ir) cells and fibers was encountered in the olfactory system of Clarias batrachus. In addition to the ir structures in the olfactory nerve, peripheral area of the olfactory bulb, and the medial olfactory tract (MOT), ir cells and fibers were prominently seen in the lamellae of the olfactory organ. Perikarya showing varying degrees of intensity of immunoreaction were observed along the base of the forebrain in the nucleus preopticus basalis lateralis, nucleus preopticus periventricularis, nucleus preopticus, nucleus lateralis tuberis pars posterior, and the pituitary. Some cells were also noticed in the midbrain tegmentum. A well-defined system of ir fibers from the MOT penetrated the telencephalon and curved dorsocaudally into the pars supracommissuralis above the anterior commissure (AC); while some fibers decussate in the AC, others extended posteriorly into the diencephalon. A fairly dense network of beaded ir fibers was seen in the basal forebrain, conspicuous around the organum vasculosum laminae terminalis and caudally traceable as far as the neurohypophysis; some immunostained fibers appear to be directly contacting with the cells of the proximal pars distalis. Fibers were also witnessed in the optic chiasma and in the inner plexiform layer of the retina. Solitary fibers were noticed in certain circumscribed telencephalic areas, caudal hypothalamus, posterior commissure, midbrain tegmentum, cerebellum, and ventral medulla oblongata. The highly organized LH-RH containing system in C. batrachus is indicative of its elaborate role in synchronization of the reproductive processes and the environmental cues.

Estrogen-2-hydroxylase in the brain of the male African catfish, Clarias gariepinus

Estrogen-2-hydroxylase activity, involved in the biosynthesis of catecholestrogens, was localized in the brain of the male African catfish, Clarias gariepinus, by means of a radiometric assay using [2-3H]estradiol as substrate. Fore- and midbrain were divided in 18, 500-microns thick, transverse sections from which small defined areas were punched out and assayed. The estrogen-2-hydroxylase activity was calculated from the release of tritium during hydroxylation, and expressed in femtomole catecholestradiol.milligram-1 tissue.hour-1. The enzyme could be demonstrated throughout the brain. A high activity (greater than 350 fmol) was observed in the telencephalon, in particularly the rostral part and the area ventralis pars dorsalis; in the diencephalon in the preoptic region, including the magnocellular part of the preoptic nucleus and the rostral part of the anterior periventricular nucleus; and in the area tuberalis, including the nucleus lateralis tuberis, the rostral part of the nucleus anterior tuberis, the caudal part of the nucleus posterior periventricularis, and in the nucleus recessus posterioris. Also a high activity was detected in the mesencephalic tectum opticum and the dorsolateral part of the torus semicircularis. The ventral mesencephalon showed a moderate (200-350 fmol) to low (less than 200 fmol) activity, whereas the lowest activity was found in the hindbrain (118 fmol). The significance of the biosynthesis of catecholestrogens in the brain is discussed in light of the negative feedback mechanism of gonadal steroids on gonadotropin release.

Differential distribution of two molecular forms of gonadotropin-releasing hormone in discrete brain areas of goldfish (Carassius auratus)

Two molecular forms of gonadotropin-releasing hormone (GnRH) were identified in the extracts of various brain areas, spinal cord and pituitary in female and male goldfish and had chromatographic and immunological properties similar to [His5, Trp7, Tyr8]-GnRH (cGnRH-II) and [Trp7,Leu8]-GnRH (sGnRH). Radioimmunoassay using different GnRH antisera after high pressure liquid chromatography did not reveal significant peaks of mammalian GnRH, [Gln8]-GnRH and [Tyr3,Leu5,Glu6,Trp7,Lys8]-GnRH in the brain extracts. The proportion of cGnRH-II-like immunoactivity to sGnRH-like immunoactivity was higher in the caudal brain areas compared to the rostral areas. The differential distribution of two GnRH forms suggest that the different GnRH forms may have different physiological functions.

Brain distribution of radioimmunoassayable gonadotropin-releasing hormone in female goldfish: seasonal variation and periovulatory changes

A radioimmunoassay (RIA) for [Trp7, Leu8]gonadotropin-releasing hormone (sGnRH) was developed to determine the gonadotropin-releasing hormone (GnRH) content in discrete brain areas of female goldfish at different stages of ovarian development. Temporal changes in serum gonadotropin (GtH) and GnRH concentrations in discrete brain areas were measured during spontaneous ovulation. There were no clear parallel changes in brain GnRH with seasonal ovarian development in goldfish. However, under a 10 degrees temperature acclimation regimen, the GnRH content in the hypothalamus and pituitary decreased as the ovary progressed from the regressed to the mature condition; on the other hand. GnRH content in the spinal cord increased in sexually mature fish compared with that in regressed fish. Significant decreases in GnRH concentration were observed in certain brain areas (olfactory bulbs, telencephalon, hypothalamus, and pituitary) of fish undergoing spontaneous ovulation compared with those of nonovulatory fish. The simultaneous changes of GnRH concentration in these brain areas suggested that the GnRH neuronal system may function as an integrated unit for the activation of GtH secretion during ovulation in goldfish.

Localization of aromatase in the brain of the male African catfish, Clarias gariepinus (Burchell), by microdissection and biochemical identification

Aromatase activity was determined in small discrete areas of the brain of the African catfish, Clarias gariepinus, by a radiometric assay. The fore- and midbrain were divided into eighteen 500-microns transverse sections. From these sections several punches (0.3 mg of tissue) were taken and incubated with [19-3H]-androstenedione. The aromatase activity, calculated from the release of tritium label during aromatization, is expressed in pmol mg-1 tissue hour-1. The highest activity (3.7 pmol) was detected in the preoptic region. The more caudally located area tuberalis, including the nucleus lateralis tuberis and the nucleus recessus lateralis, also showed a relatively high activity (2.5 pmol). A similar activity was found in the most rostral part of the telencephalon and the dorsal parts of the mesencephalon, i.e., the tectum opticum and torus semicircularis (2.3 pmol). A moderate aromatase activity was observed in remaining parts of the brain, except the cerebellum and hindbrain, in which aromatase activity was hardly detectable (0.1-0.3 pmol). It is concluded that a high aromatase activity is present in regions known to be involved in the regulation of reproduction. Since both the torus semicircularis and the tectum opticum display a high aromatase activity, it is suggested that also these structures are involved in reproductive processes.

Relationship between brain gonadotrophin-releasing hormone (GnRH) and seasonal reproductive cycle of "caribe colorado," Pygocentrus notatus

Immunoreactive gonadotrophic hormone-releasing hormone (ir-GnRH) was detected in hypothalamic and telencephalic extracts of the Venezuelan freshwater fish "caribe colorado," Pygocentrus notatus. Hypothalamic ir-GnRH from female fish demonstrated displacement curves parallel to those of synthetic mammalian luteinizing hormone-releasing hormone (LHRH). The content of hypothalamic and telencephalic ir-GnRH from female fish was more than four-fold greater than that of male animals. Also, fluctuations that depended on the reproductive state and environmental conditions (rainfall) occurred in females but not in males. Thus, ir-GnRH levels were higher in hypothalamic and telencephalic extracts from sexually mature females than in those from fish sampled outside the climatically determined breeding season.

Central regulation of reproduction in teleosts

As in other vertebrates, reproduction in teleosts depends upon interactions taking place along the brain-pituitary-gonads axis. At the central level, these interactions involve at least three types of factors:A gonadotrophin-releasing factor which has recently been isolated from chum salmon brain extracts. This decapeptide, whose structure is (Trp(7)-Leu(8))-LHRH, appears to have a widespread distribution among teleosts, and is less active that LHRH or LHRH analogues in releasing gonadotrophin from the teleost pituitary. Immunohistochemical and quantitative studies have demonstrated that Gn-RH neurons are mainly located in the ventral telencephalon and the preoptic area, while projections are found in the entire brain and the pituitary gland.A gonadotrophin release-inhibiting factor has been demonstrated in the anterior preoptic region of the goldfish and a large set of data suggests that dopamine has GRIF activity in goldfish, and in other teleost species, by direct action on the gonadotrophs. Accordingly, a dopaminergic preoptico-hypophyseal pathway could be demonstrated in the goldfish brain.Sex steroids exert, depending on the dosages, either a negative feedback in sexually mature fish or a positive feedback in immature fish. Such a positive feedback is caused by estrogens and aromatizable androgens. Accordingly, the brain of teleosts contains high levels of aromatase activity in particular in the telencephalon and anterior hypothalamus. The distribution of estrogens concentrating cells within the brain is consistent with possible interactions with Gn-RH or catecholaminergic neurons at the level of certain brain territories.These data are discussed in relation with the functional significance of different brain areas where interactions between these different factors possibly take place, in particular the terminal nerve, the ventral telencephalon, the preoptic area and nucleus lateralis tuberis.

A reinvestigation of the Gn-RH (gonadotrophin-releasing hormone) systems in the goldfish brain using antibodies to salmon Gn-RH

The organization of Gn-RH systems in the brain of teleosts has been investigated previously by immunohistochemistry using antibodies against the mammalian decapeptide which differs from the teleostean factor. Here, we report the distribution of immunoreactive Gn-RH in the brain of goldfish using antibodies against synthetic teleost peptide. Immunoreactive structures are found along a column extending from the rostral olfactory bulbs to the pituitary stalk. Cell bodies are observed within the olfactory nerves and bulbs, along the ventromedial telencephalon, the ventrolateral preoptic area and the latero-basal hypothalamus. Large perikarya are detected in the dorsal midbrain tegmentum, immediately caudal to the posterior commissure. A prominent pathway was traced from the cells located in the olfactory nerves through the medial olfactory tract and along all the perikarya described above to the pituitary stalk. In the pituitary, projections are restricted to the proximal pars distalis. A second immunoreactive pathway ascends more dorsally in the telencephalon and arches to the periventricular regions of the diencephalon. Part of this pathway forms a periventricular network in the dorsal and posterior hypothalamus, whereas other projections continue caudally to the medulla oblongata and the spinal cord. Lesions of the ventral preoptic area demonstrate that most of the fibers detected in the pituitary originate from the preoptic region.

Immunoreactive gonadotropin-releasing hormone-like material in the brain and the pituitary gland during the periovulatory period in the brown trout (Salmo trutta L.): relationships with the plasma and pituitary gonadotropin

In fish there are few data on the gonadotropin-releasing hormone (Gn-RH) neurosecretory activity, which could explain long- and short-term variations of the gonadotropin secretion. There is no biological species specificity between mammal and fish Gn-RH; although there is a structural difference, they are, on the contrary, characterized by a high immunological specificity which does not allow measurement of fish Gn-RH using radioimmunoassay for LH-RH. We have synthesized salmon Gn-RH according to the formula recently proposed by Sherwood (N. Sherwood, L. Eiden, M. Brownstein, J. Spies, J. Rivier, and W. Vale, 1983. Proc. Natl. Acad. Sci. USA 80, 2794-2798). Its activity has been tested by its ability to stimulate the gonadotropin hormone (GtH) secretion in vivo in testosterone-implanted juvenile rainbow trout, and for the recognition of synthesized Gn-RH (s-Gn-RH) perykaria by a specific antibody raised against the s-Gn-RH in regions of the brain described as containing LH-RH immunoreactive-like material. A radioimmunoassay has been developed for the salmon Gn-RH, and its specificity to measure trout Gn-RH has been tested. Using this assay, the brain and pituitary Gn-RH contents have been measured throughout the final phases of maturation and ovulation. Brain Gn-RH increases from the end of vitellogenesis (8.9 +/- 0.76 ng/brain) to ovulation (more than 15 ng/brain). Pituitary Gn-RH is lower (1.58 +/- 0.69 ng/pituitary) at the end of vitellogenesis and follows a similar profile as in the brain, except for a significant decrease just prior the beginning of oocyte maturation. The correlations between Gn-RH levels and GtH pituitary and plasma levels show that total brain Gn-RH is never correlated to the GtH, suggesting that the increase in the brain Gn-RH content is related to a Gn-RH system closely related to maturation and ovulation, which remains to be investigated. On the contrary, pituitary Gn-RH levels are well correlated with pituitary and plasma GtH levels, indicating that pituitary Gn-RH levels might represent a good index of the Gn-RH neurosecretory activity in the fish hypothalamohypophysial complex, given the absence of a portal system in teleost.

Occurrence of beta-endorphin-like immunoreactivity in the brain of the teleost, Boops boops

By immunocytochemical methods the present study describes beta-endorphin-like immunoreactive substance in the brain of Boops boops. Beta-Endorphin-like neurons and fibers were detected in both dorsal and ventral components of the preoptic nucleus and in the nucleus lateralis tuberis. This localization has been discussed in relation to the presence, in the same area, of a well-defined neurosecretory system involved in hypophysial regulation. A beta-endorphin-like immunoreaction was also detected in the Purkinje cells and in processes within the cerebellum molecular layer. Although this last finding remains enigmatic it may suggest a neuromodulatory activity for cerebellum beta-endorphin-like substance. No immunoreaction was observed when the specific antiserum was absorbed with corresponding antigen and with beta-LPH. These tests led to the conclusion that the immunostaining reaction might correspond to a beta-endorphin- or lipotropin-like reaction. Further, the present results show the phylogenetic antiquity of a beta-endorphinergic or lipotropinergic system in the brain, with a stable evolutionary history as a hypophysial regulatory factor or neuromodulatory agent.

Efferents from the supracommissural ventral telencephalon in the hime salmon (landlocked red salmon, Oncorhynchus nerka): an anterograde degeneration study

Efferents from the area ventralis telencephali pars supracommissuralis (Vs) of hime salmon (landlocked red salmon, Oncorhynchus nerka) were studied using anterograde degeneration methods. It was found that Vs sends intratelencephalic fibers bilaterally to the internal cell layer of the olfactory bulb and wide areas of telencephalon, while it sends extratelencephalic fibers bilaterally to habenulae, inferior lobes and midbrain tegmenti through medial and lateral forebrain bundles and lateral preoptic area. A possibility was pointed out that some of these extratelencephalic projections are involved in the pathway controlling the sexual behavior which are suggested from behavioral experiments.

Immunocytochemical identification of CRF-like and SRIF-like peptides in the brain and the pituitary of cyprinid fish

Single and double immunocytochemical techniques were applied to the brain and pituitary of carps and goldfish. With the use of antiserum raised against synthetic corticotropin-releasing factor (CRF), immunoreactive perikarya were observed in the nucleus praeopticus and the nucleus praeopticus periventricularis. CRF-like-immunoreactive hypothalamic nerve fibers reach the pituitary. In cyprinids, some fine fibers enter the rostral neurohypophysis bordered by prolactin- and ACTH cells. Other thicker fibers extend ventrocaudally into the neurointermediate lobe. This CRF-like system appears to differ from the SRIF-like system, which is restricted to the proximal pars distalis of the pituitary containing somatotrophs.